This project evaluated the use of CD69 expression as an indicator of T cell activation following polyclonal T cell activation using mitogens phytohemagglutinin(PHA) and concanavalin A (conA) and monoclonal antibodies (CD2/CD2R) as activators. CD69 expression was shown to be an indicator of T cell activation after T cells are activated with mitogens (PHA, ConA, and interleukin 2/2R (1L2/IL-2R)). The expression of CD69 was compared with other activation antigens including CD25, CD71, and human lymphocyte antigen-DR at various times after activation, as well as with tritiated thymidine incorporation at the end of the culture. CD69 was the earliest antigen expressed and reliably predicts T cell activation at 4 to 6 hours. However, to evaluate recall antigen induced activation, the expression of CD69 cannot be assessed until 24 hours and is optimal at 48 hours. Finally, the best approach to evaluating CD69 expression is to assess increased fluorescence rather than to identify the percentage of cells expressing CD69.